1. Field of the Invention
This invention relates to a process for the synthesis of quaternary ammonium hydroxides by an electrolytic process.
2. Description of the Related Art
Quaternary ammonium hydroxides are currently used industrially as support electrolytes, as stabilisers or solubilisers for organic compounds in aqueous solutions in electrochemical processes, as microbicides, as template agents in the synthesis of numerous zeolites, etc.
The direct synthesis of quaternary ammonium salts is normally effected by a reaction of addition starting from the corresponding trialkylamine and the alkyl sulphate, carbonate or halide. The alkyl sulphates and carbonates, and in particular methyl and ethyl sulphates, are compounds of high teratogenic activity.
In contrast the halogenated compounds are relatively harmless. In addition, the alkyl chlorides, bromides and iodides add extremely easily to trialkylamines, to form the corresponding quaternary ammonium salts.
The methods mostly used for converting the salt to the hydroxide are: reaction between the halide and the hydrated oxide of a metal of which the halide is insoluble in water (Ag.sub.2 O, HgO) as described for example in S.U. Pat. No. 662,547; exchange over cation resin as described in U.S. Pat. No. 3,579,581; or exchange against KOH in alcoholic solutions of tetraalkylammonium chlorides or bromides (ANAL. CHEM. vol. 34, 1962 page 172).
In preparing Ag.sub.2 O(H.sub.2 O) and HgO(H.sub.2 O), alkalis (NaOH, KOH) are used and these become strongly absorbed by the hydrated oxide. Elimination of the alkaline residues requires considerable time and wash water, otherwise an ammonium base is obtained containing high concentrations of undesirable alkaline metals. The overall process, considering the high cost of the metals used, is not economical for the industrial production of quaternary ammonium bases. In the other process, using anion exchange resins, the low exchange capacity of the resins leads to low ammonium base productivity per passage and per unit of resin volume. Moreover, the complexity of the operations involved in regenerating the spent resin, the large quantities of solutions necessary for effecting the regeneration, and the difficulty of disposing of the liquid effluents which are contaminated with the ammonium base and with products originating from the utilised resins make this process difficult to carry out on an industrial scale. Finally, exchanging the halide in alcoholic solution against alkaline bases does not enable alkali-free ammonium bases to be obtained because the alkaline halides have a certain solubility in alcoholic solutions.
Methods based on electrolysis have recently been proposed for exchange reactions. This relatively new technology has received a considerable impulse from the study and consequent availability of new semipermeable ionic membranes. U.S. Pat. Nos. 3,402,115 and 3,523,068 describe two processes which use electrolytic cells. The described methods are based on the synthesis of hydroxide from quaternary ammonium salts which have a non-electrolysable anion (sulphate, nitrate, carbonate). The method of the first patent (U.S. Pat. No. 3,402,115) is based on the use of an electrolytic cell (FIG. 1) comprising three compartments separated by two membranes, of which one is an anion exchange membrane (1) and one a cation exchange membrane (2). Initially an acid solution (using sulphuric acid) is fed into the anode compartment, the quaternary ammonium salt solution is fed into the central compartment and a very dilute aqueous solution of quaternary ammonium hydroxide is fed into the cathode compartment. On passing current, the cation Tetra Alkyl Ammonium (TAA.sup.+) (3) migrates towards the cathode through the cation exchange membrane, and the anion (4) migrates towards the anode through the anion exchange membrane. H.sub.2 (5) develops in the cathode zone to form a TAAOH solution, whereas O.sub.2 (6) develops in the anode zone to form an acid solution.
The method of the second patent (U.S. Pat. No. 3,523,068) is based on the use of an electrolytic cell (FIG. 2) comprising two compartments separated by a cation exchange membrane (2). Initially the anode compartment is fed with an acid solution of a quaternary ammonium salt having an anion which does not discharge by electrolysis, and the cathode compartment is fed with distilled water. On passing current the cation (3) passes into the cathode compartment where H.sub.2 (5) develops to form a TAAOH solution, whereas O.sub.2 (6) develops in the anode compartment and the pH reduces.
The limits of these methods are: the need to use quaternary ammonium salts other than halides, and which have to be synthesised from alkyl sulphates (of which the noxiousness has already been mentioned); the presence of highly acid solutions which lead to corrosion resistance problems for the materials of construction; and the use of cation membranes which have to be highly selective towards the alkyl ammonium group, so that each alkyl ammonium salt requires an appropriate type of membrane according to the composition of the organic part.
These membranes also allow passage of undissociated salt towards the cathode, and thus do not ensure a high exchange yield, and the product obtained does not have the required high purity characteristics. In this respect, in the process described in U.S. Pat. No. 3,523,068, the base obtained is then purified by passage over anion exchange resin, and this leads to the stated drawbacks of the relative process. (Eisenhauer et al. U.S. Pat. No. 3,523,068 page 4 line 35).
Finally, in electrolytic processes based on the migration of the alkyl ammonium cation, it has been observed that this latter migrates together with numerous molecules of solvation of H.sub.2 O, this constituting an obstacle in obtaining concentrated base solutions.
A process has now been surprisingly found for the synthesis of quaternary ammonium hydroxides, which uses substances of low noxiousness, is simple to operate, and is of low plant and running costs. The quaternary ammonium hydroxides produced by this process have a low production cost and a high purity level, in contrast to the quaternary bases produced by known processes, which are of modest purity and high production cost.